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Old 09-04-2010, 10:35 AM   #1
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How does my Heat Pump/Furnace Work?

First of all, I'm very familiar with how a propane furnace works, but have never had a heat pump in an RV. I did a search of the forum, but nothing jumped out at me to answer my basic questions.

In looking at all the literature that came with our new coach (see sig below), I have an Atwood Hydro Flame Furnace manual, a Suburban Gas Furnace manual, and an Airxcel Roof Top Heat Pump and Ceiling Plenums manual. The furnace cover on the curb side of the coach says Suburban. My thermostat says RV Comfort.HP (I believe that is also associated with Suburban). So I'm not sure why I was given the Atwood Hydro Flame manual. I believe my water heater is an Atwood, but the manual specifically says "furnace" on it. I can't find a manual for the water heater in the stuff they gave me.

My Excel Owner's Manual (the Component Warranty Summary page) lists the furnace as a Hydroflame (Atwood???), and the Air Conditioner as a Coleman. There's no manual for the thermostat or the air conditioner. So I'm not really sure what I have! Not a big deal; I just want to know how the heat pump vs. the gas furnace work.

My thermostat has three switches; the one in question is the "Cool/Off/Gas Heat/Elect Heat" switch. The temps here in the high desert are dipping down into the 50s at night. Jan is comfortable in the 80s and above range. So to take the chill off in the mornings, I've been turning on the "Elect Heat". It sounds like the A/C comes on, then immediately switches to the heat mode. Sometimes the warm air comes from the ceiling vents, sometimes from the floor vents (I still have to confirm this). The "Hi/Lo" Fan Speed switch doesn't affect the speed of the blower (like it does with the A/C). I guess that's normal. However, when I put my hand in front of the Suburban vent on the outside of the coach, it's hot. I thought that was only the gas heat appliance. If I have the thermostat on Elect Heat, why would that have anything to do with the Suburban heater? And then, after a while, the fan speed automatically speeds up for a while until the thermostat temperature is reached.

Anyone know if all of this is normal and I should just not worry about it? Or is something not working correctly? Thanks!!
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Old 09-04-2010, 01:43 PM   #2
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I can't answer most of your questions..but will try to share what I know. We have the Coleman Mach normal AC and the big 43,000 BTU Suburban dual stage gas furnace. Suburban is new to PI as they have only been installing them for about a year. My experience with my manuals that come with the rig are usually what comes as standard equipment. I have manuals of stuff I don't have on my rig and also lacking manuals on what I do have. E.I. I have manuals on outfitter rear jacks, old model Coleman-Mach AC and etc. You need to get the correct manual for the Suburban. Good luck as I have been trying to get a manual for my new style Coleman-Mach AC (no chille grill) to new avail. The Suburban is a very different animal then the Atwood furance. You are correct you have a Atwood water heater.

When your thermostat is on "auto" the thermostat picks the blower speed it wants (usually goes to low blow if it's within about 4* demand and will go to high if more than 4* difference), thus the HI/LO switch does nothing. Only when the switch is turn to "ON" do you have fan control.

Regarding the issue on the gas furnance being hot........I'm wondering if this works similiar to a home heat pump unit. It uses the heat pump up to a certain temp (in homes about 45-50*F) and then the gas furnace comes on to help out the HP. Just a guess.

I don't know if I help or not..... rockin'
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Old 09-04-2010, 06:34 PM   #3
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I will make it as easy as I can... Do you know how an air conditioner works (Technically a furnace is an air conditioner by the way but I'm talking about the one in/on the roof that cools the air)

Now, imagine it was installed upside down, with the cold air going outside and the hot air being blown around inside.

Ok.. That's a heat pump.

The difference between a heat pump and an air conditioner is that the A/C pumps heat ONE WAY (out of the motor home)

The heat pump has a "Reverse" switch (Marked HEAT) that pumps the head INTO the motor home.

Now.. Most heat pumps have issues if the temp drops much below a set point, 35-40 is a common point, hopefully the unit has a sensor to take care of that. (Shuts it off and fires up the furnace instead)

That is the very basic explanation.

If you want more details.. I got those too.. but a refrigeration engineer.. I'm not so I prefer to leave the high tech stuff to them.
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Old 09-06-2010, 09:07 AM   #4
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I will make a stab at all this:

From what I understand__AIRXCEL is now the parent owner of both RV Products (Coleman) and Suburban. So manuals and names can get confusing.

I don't have a heat pump unit and without knowing the model numbers for the equipment...will only provide a link that might be helpful.
Atwood water heater
RV Products: TSTAT manuals
AIRXCEL site map

As far as the operation of the Heat function on the unit.....it is my impression that when using the heat pump to remove the chill or for heating....the heat pump will provide the heating for the unit (RV). If the actual temperature inside and sometimes outside temperatures being to low... the unit and the 'set point' or demand is to far apart....the TSTAT will fire up the gas furnance to provide additional heat to suppliment the heat pump.

Might try setting the 'demand' heat a few degrees higher than the actual inside temp and see if the heat pump runs along and them set the 'demand' heat much higher than the inside temp and see what happens.

But it sounds like it might work that way....not having a heat pump or TSTAT model, I can only go by what I've found so far. The TSTAT info will provide the answer.

Hope something helped.

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Old 09-06-2010, 10:48 AM   #5
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Thank everyone. This morning, I was able to confirm that sometimes the warm air comes out of the floor vents and sometimes out of the ceiling vents, very odd. Thanks for the links, Work. I'll have to do some reading to see if I can figure it out. I don't think there's a problem; I just like knowing how things work!
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Old 09-06-2010, 10:51 AM   #6
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Quote:
Originally Posted by gdbontly View Post
Thank everyone. This morning, I was able to confirm that sometimes the warm air comes out of the floor vents and sometimes out of the ceiling vents, very odd. Thanks for the links, Work. I'll have to do some reading to see if I can figure it out. I don't think there's a problem; I just like knowing how things work!
Oh-oh.....we may have another SKIA on board!!! rockin'
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Old 09-06-2010, 11:11 AM   #7
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Old 09-26-2010, 07:11 AM   #8
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Well, I think I finally understand how it all works. I spent some time on the rvcomfort.com site and found that the DON'T have a copy of the manual for my particular thermostat (RV Comfort.HP, Model 9530-335). I called them about a week ago, and they said they would send me a .pdf file, but I haven't received it yet. BUT, some of the other theremostats work the same with respect to how the "heat pump brings on the gas furnace as a backup heat". It's quite a complex algorithm! So here's the "Reader's Digest" version:

If you set your thermostat within 3 degrees of the room temperature (e.g., thermostat says 70 and you raise it to no more than 73), and turn on Electric Heat, the heat pump will come on and the warm air will come out of the ceiling vents. If you set the thermostat MORE than 3 degrees, the gas furnace will come on instead, and the warm air will come out of the floor vents.

For more details, please see the attached excerpt from the Model 6536-335 thermostat. Excerpt from 6536-335 thermostat manual.pdf
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Old 09-26-2010, 09:08 AM   #9
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Glenn what you say make perfect sense to me. Obviously not as sophisticated as a home unit. rockin'
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Old 09-27-2010, 06:13 AM   #10
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Originally Posted by gdbontly View Post
If you set your thermostat within 3 degrees of the room temperature (e.g., thermostat says 70 and you raise it to no more than 73), and turn on Electric Heat, the heat pump will come on and the warm air will come out of the ceiling vents. If you set the thermostat MORE than 3 degrees, the gas furnace will come on instead, and the warm air will come out of the floor vents.

For more details, please see the attached excerpt from the Model 6536-335 thermostat. Attachment 5712
When you set it greater than 3*, does the gas turn off and the electric come on when within 3* of the set temp?
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Old 09-27-2010, 09:48 AM   #11
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When you set it greater than 3*, does the gas turn off and the electric come on when within 3* of the set temp?
I haven't confirmed this yet, but according to the instructions, it will switch back and forth between electric and gas for three cycles, and then the electric heat will go into a lock down mode for 1 hour + 45 minutes. Right now, the weather isn't cold enough here to test it out.
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Old 09-28-2010, 08:03 AM   #12
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When you set it greater than 3*, does the gas turn off and the electric come on when within 3* of the set temp?
I tried it this morning, and no, it didn't. The room temp was at 65 degrees according to the thermostat. I set the thermostat to 68 and the heat pump turned on. Then I raised it to 70 (more than 3 degrees above room temp) and it automatically switched to the gas furnace. As the furnace raised the room temp to within 3 degrees of the set temp, it did NOT switch back to the heat pump. The furnace continued to run until the set temp was reached, and then shut off.

I'm just trying to determine how to make maximum use of the electric heater so I don't have to fill my propane tanks as often this winter. So I guess I'll just have to to my best to keep the set temp within the 3 degrees of the room temp.
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Old 09-28-2010, 08:23 PM   #13
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After the furnace got it up to 70* did the heat pump take over maintaining the temp when it started to get cold again?
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Old 09-28-2010, 09:30 PM   #14
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Your heat pump/AC is operating in the normal manner. As mentioned earlier the "heat pump" is a 2 way air conditioner. It has both an evaporator and a condensor. When in the AC mode the compressor converts the cool low pressure gas comming from the evaporator (inside cooling unit) to a hot high pressure gas. The high pressure gas is then run through the condensor (outside heat disipating unit) to turn the gas back into a liquid. The high pressure liquid then runs through an orfice before rentering the evaporator. As it runs through the orifice the liquid expands. As it expands from a liquid to a gas it cools. The now cool gas runs through the evaporator. A fan blows air across the cool evaporator coils and the air is cooled as it enters the living quarters.

When in the Heat mode the thermostat energises a switching valve that runs the Freon in the opposite direction through the system. The outside condensor (heat disipator) now becomes the evaporator (cooling unit) and vice versa. The cool air is disipated outside and the warm liquid freon is run through the inside. As the fan blows air across the warm coils the heat is transfered into the living quarters.

The lower the outside temperature the less effective the unit is as a heat pump. That's why most units don't work below about 37*F. In our case the gas furnace "assists" the heat pump if the set temperature is more than 4*F below the ambient indoor temperature and over 37*F outside. The gas furnace will blow heat through the floor registers and the heat pump will blow warm air through the ceiling vents. Once the set temp is met the gas furnace will shut off and the heat pump will maintain the set temp.

If the outside temp is less than 37*F the heat pump will attempt to run for 3 cycles. If the outdoor sensor determines the outdoor temperature is cold enough to frost over the cooling coils it shuts down the unit. The thermostat will allow the heat pump to attempt 3 starts before it will turn it off and the gas furnace will take over.
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